1. Field of the Invention
The invention relates to a method for automatically correcting torque for substituting a permanent-split capacitor (PSC) motor by an ECM.
2. Description of the Related Art
A PSC motor having a stable rotational speed for a civil HVAC system is shown in FIG. 1, which takes an indoor unit of the HVAC system as an example. The motor includes four winding gear taps for modulating rotational speeds of two heating fans and two cooling fans, respectively. The rotational speed of the fan is controlled by the control board of the stove. The control board is provided with cooling/heating relay, low/high cooling relay, and low/high heating relay. Other HVAC systems comprise two heating stages and a single cooling stage, or a certain combination of the heating rotational speed and the cooling rotational speed.
The PSC has a reasonable efficiency when it runs at a high rotational speed, and the efficiency thereof decreases to below 20% when it runs at a low rotational speed. As the air conditioner evaporator coil necessitates a higher air flow than the stove heat exchanger, the motor of the blow operates at a relatively low rotational speed during the operation of the stove, thereby being poorly efficient. When the control board of the stove is provided with a special relay for controlling a continuous fan mode, the motor operates at a much lower rotational speed during the “ON” operating period of the continuous fan mode, the efficiency is the lowest.
Because the PSC motor has a low efficiency, most updated HVAC systems utilizes electronically-commutated motor (ECM), also called DC blushless permanent magnet synchronous motor. The power of the ECM motor is approximately proportional to the cube of the rotational speed, while the power of the PSC motor is approximately proportional to the rotational speed. Thus, when the rotational speed of the motor decreases, the power of the ECM motor is lower than the power of the PSC motor within the whole range of the rotational speed of the motor. This is very important when blower continuously operates.
It is desired that the ECM motor is directly used to substitute the present PSC motor without changing the original arrangement of wires and connection relationship of the original HVAC system, realizing the advantage of the ECM motor. The replacement is also advantageous in that the simple control circuit is used, thereby decreasing the complexity of the substituting system and eliminating additional arrangement of wires.
A typical ECM motor for substituting the PSC motor is shown in FIG. 2. The ECM motor includes a motor entity 1 and a motor controller 2. The motor entity 1 comprises: a stator assembly; a rotor assembly; a housing assembly, and 6 gear input lines L1, L2, L3, L4, L5, and L6. The motor controller 2 includes a control circuit board and a box. The motor controller 2 has a structure as shown in FIG. 3 and includes: a microprocessor, an inverter circuit, a gear detection circuit, an operating parameter detection unit, and a power supplier. The operating parameter detection unit detects a rotational speed signal of the rotor, a phase current signal, a phase voltage signal of the motor. The power supplier comprises a rectification circuit, a voltage stabilizing circuit, and a DC-DC conversion circuit. The gear detection circuit detects the energizing state of six gear input lines L1, L2, L3, L4, L5, and L6. Six rotational speeds (V11, V12, V13, V14, V15, and V16) are preset by the microprocessor in ascending order to corresponding to the six gear input lines L1, L2, L3, L4, L5, and L6. One or more (generally no more than four) of the six gear input lines are in an activated and efficient state, but only one rotational speed is selected via the logic relationship of the gear input line. The motor is controlled to operate at the six rotational speeds (V11, V12, V13, V14, V15, and V16) in a constant torque mode. Because the torque is proportional to the current, the control of the rotational speed of the motor is virtually converted into the control of the torque, that is, the control of the current. Six torque data (T11, T12, T13, T14, T15, and T16) corresponding to the six rotational speeds (V11, V12, V13, V14, V15, and V16) are stored by the microprocessor as the factory defaults of the motor.
However, because with the continuous change of the application environment of the motor, for example, the requirements of the static pressure and the power, the factory default of the set of the torque data corresponding to the six gear input lines are possibly not suitable to the environment, and the torque is required to be corrected for meeting the requirement of the application environment, thereby making the actual rotational speed be comparable with the expected rotational speed. A typical method for torque correction includes: operating the motor in the working field; allowing the motor to meet requirements of five rotational speed (V11, V12, V13, V14, and V15); recording actual measured torque data (T110, T120, T130, T140, and T150); and replacing the preset torque data (T11, T12, T13, T14, and T15) of the factory default with the actual measured torque data (T110, T120, T130, T140, and T150) for operating the motor.
The above torque correction method has the following disadvantages: 1) a narrow application range. Because a present PSC motor has two typical rated rotational speeds, one is a 1080 RPM motor having 6 poles, an actual rated rotational speed is between 1000 RPM and 1200 RPM; and the other is an 825 RPM motor, an actual rated rotational speed is between 800 RPM and 875 RPM. When six rotational speeds are designed as a set by the microprocessor, the highest rotational speed is 1075 RPM as disclosed by the prior arts. Therefore, the other four rotational speeds are designed between 600 RPM and 1075 RPM, the gap between every two rotational speeds is very large; besides, rotational speed below 600 RPM cannot be obtained in a continuous fan mode, the two rated rotational speeds are not met, thereby resulting in the low application range. 2) the torque is unable to be fine-tuned. Because only one set of actual measured torque data are stored in the microprocessor, when the external static pressure or the environment changes, no other set of actual measured torque data can be selected to adapt to the external environment, resulting in inaccurate control. In substituting the PSC motor, when the external static pressure is relatively high due to the accumulation of dust in the air channel, the steady torque of the motor in the mode of the constant rotational speed of 1075 RPM will be smaller than the standard steady torque provided by the manufacturer, and in normal operation of the motor, the actual torque will be relatively smaller, thereby resulting in a smaller air volume.